Porous electrode primary battery having state of charge sensing electrode

ABSTRACT

In a porous electrode primary battery a sensing grid is positioned in a cell on or near the surface of the porous cathode facing the separator and anode. The voltage measured between this sensing grid and the conventional cathode current collector grid is a function of the current distribution within the electrode which is continuously changing as the battery discharges, thus the measured voltage is indicative of the state of charge of the particular cell having the sensing grid and for a battery containing cooperatively connected cells, the state of the battery in general.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured or used by or for theGovernment of the United States for all governmental purposes withoutthe payment of any royalty.

BACKGROUND OF THE INVENTION

The field of the invention is in the primary battery art, and moreparticularly in the art of porous electrode primary batteries.

In the absence of good history records, it has been quite difficult todetermine the present condition of a primary battery with respect to itsremaining useful life. Generally, the voltage and currentcharacteristics of a battery with, for example, half its useful lifeused up are substantially the same as a new battery. With secondarybatteries, which can be recharged, the necessity of knowing their stateof charge is generally not so important as they can be readily rechargedto substantially a known energy content.

The best known prior art is that of U.S. Pat. Nos. 2,988,590 to patenteeH. G. Andre, and 3,720,869 to patentee Rowlette.

SUMMARY OF THE INVENTION

An improved primary battery structure is disclosed which provides meansfor making a simple voltage measurement by which the remaining usefullife of the battery may be ascertained.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic-pictorial representation of an improved cellillustrating the invention;

FIG. 2 schematically illustrates the effective active area in thecathode of a new cell;

FIG. 2a schematically illustrates the cathode of FIG. 2 afterapproximately half of the useful life of the cell is used;

FIG. 2b schematically illustrates the cathode of FIG. 2 afterapproximately three-fourths of the useful life of the cell has beenused;

FIG. 3 schematically illustrates the current distribution and voltagemeasurement in a cell having approximately half its useful life used;

FIG. 4 pictorially represents a side view of an improved cathodeincorporating the structure of the invention;

FIG. 5 is a side view of the cathode illustrated in FIG. 4;

FIG. 6 is a top view of the cathode illustrated in FIG. 4;

FIG. 7 is a top view illustrating an alternative location of the voltagesensing grid; and

FIG. 8 is a top view illustrating another embodiment of the inventionhaving a first and a second state of charge sensing grids.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally, the term battery is used to designate a chemical source ofelectrical energy. It is composed of one or more cells arranged,connected, and cased with means for external electrical connection. Asingle cell, having terminals for exterior connection is frequentlycalled a battery interchangeably with the term cell. Generally, thestate of charge of any cell in a battery is representative of thebattery as a whole. Thus, while this invention measures the state ofcharge of a single cell, with a sensing grid, it is applicable to thecharge of the battery of which the cell is one of a group of cells.Obviously, if desired, more than one or even all the cells in a batterymay be fabricated with a sensing grid or electrode.

A typical embodiment of the invention as illustrated in FIG. 1represents an improved Lithium Inorganic cell. Typically, several cellsare fabricated into a battery, and typically each electrode serves bothadjacent electrodes, i.e., porous carbon cathode 11 serves both lithiumanodes 12 and 13, and lithium anode 12 serves cathodes 11 and 14. Anindividual cell can be considered extending from the collecting grid ofone electrode to the collecting grid of an adjacent opposite polarityelectrode, (with generally a separator in between). With thisconventional type of structure the current collecting grids for theelectrodes are positioned in the center of the electrodes. Thus, grid 15with connecting tab 16 is centrally located in the double cathode 11,and anode grid 17 with connecting tab 18 is centrally located in doubleanode 12. Nickel is the conventional material from which to fabricatethe collecting grids. Conventional glass mat separators 19, 20, and 21isolate the electrodes and position them in cell case 22. Lithiuminorganic cells and their fabrication are well known in the art. (Forsingle cell batteries, in flat cell construction the collecting gridsare positioned in their respective electrodes at the surfaces of theelectrodes opposite the common separator. In round cell construction,the connector for the center electrode is in the center of the cell andthe connector for the concentric outer electrode is at the outerperiphery of the cell.)

In the operation of porous electrode primary batteries, as the celldischarges the apparent source of the current in the cathode movessequentially from the cathode surface next to the separator (adjacentthe anode) in toward the cathode current collector grid. This is due tothe salting out of insoluble discharge products which occurs initiallynear the face of the cathode facing the anode and continues until thatarea is blocked. The apparent current source moves just ahead of theblocked area and both the source and the blocking proceed toward thecathode current collector grid until the complete cathode becomesblocked or the anode active material is exhausted or the catholyte isexhausted and the cell ceases to function. This blocking action on thecathode is schematically diagrammed in FIGS. 2, 2a, and 2b. In FIG. 2, anew battery, the apparent source of the current in the cathode betweenthe cathode 30 and anode 31 is adjacent the cathode surface indicatedbetween the arrows 32 and 33, for the right hand cell. For the left handcell the apparent current source from cathode 30 to anode 34 is betweenarrows 35 and 36. After the battery has been used for a period of timethe cathode 30, as illustrated in FIG. 2a, becomes progressivelyblocked, 37 and 38, inward from its outer surfaces and the apparentsources of the currents move inward ahead of the blocked region to theregions indicated between the arrows 39-40 and 41-42. With further useof the battery the blocking and current sources move on toward thecollecting grid 45 as illustrated in FIG. 2b.

I have found that foregoing action is generally true of all porouselectrode primary batteries. I have also found that by placing a sensinggrid in a porous cathode near the surface next to a separator thatseparates the cathode from an anode that the extent of this blocking maybe determined by the determining the location of the apparent source ofcurrent in the cathode. This apparent source of current is directlyrelated to the remaining life of the battery. My invention comprisesforming a sensing grid 50 in cathode 11 as illustrated in FIG. 1. Thesensing grid 50 is positioned in the cathode 11 near the surface of thecathode 11 adjacent the separator 20. Thus, the actual cathode for thiscell is substantially bounded by the sensing grid on the interior sideof the cathode and the collection grid at the other, or opposite, sideof the individual cell cathode. A conventional electrical connection tab52 is attached to the grid 50. As energy is taken from the battery inthe form of an exterior current flow between connections 16 and 18 aninternal current flow takes place within the cell of the battery fromthe apparent source location in the cathode, across separator 20, to theanode. This is schematically diagrammed for partially discharged("used-up") cell in FIG. 3. The cathode 60 has collecting grid 61 andthe anode 62 has collecting grid 63. Separator 64 separates the anodeand cathode. The sensing grid 65 is located in the cathode as previouslydescribed. This particular cell illustrated is partially salted andblocked so that the apparent current source in the cathode 60 is betweenthe arrows 66 and 67. I have found that there is a small but easilymeasured voltage drop across the cathode between the apparent source ofcurrent and the conventional cathode current collecting grid. Thus, inFIG. 3 the current flow in the cathode 60 is from the apparent source66-67 through the remaining substantially inactive cathode region,between the source and the collecting grid 61. This is represented byresistance 70. The "used-up" portion of the cathode between the source66-67 and the surface next to the separator 64 is inactive in thegeneration of current and hence substantially no cell output currentflow is present in this part of the cathode and no voltage drop occursin this region. (Current does flow between 66-67 and the anode 62through the electrolyte and separator, however.) Thus, a voltmeter 71connected between the conventional cathode current collecting grid 61and the sensing grid 65 will read a maximum voltage drop, i.e., acrossthe total cathode of a cell for a new cell and a minimum ofsubstantially zero volts for a substantially fully discharged cell (thatis cathode limited), with appropriate in between values of voltagerepresenting the useful life remaining in the cell. In cells that aredesigned to be limited other than by cathode blockage, the end of thecell life will occur at a determined voltage above zero. For instance,in a particular cell that is designed to be anode limited, that is, allthe active anode material will be used up before the cathode becomescompletely blocked, the range of sensing voltage is from 50 μV for afully charged (new) cell to approximately 10 μV, at which time the anodeis on the verge of being substantially all used uup. Ideally for aperfect cell, it would quit all over at the same time, i.e., the cathodebecomes completely blocked just as the last bit of active anode materialand catholyte material is exhausted. However, due to the hazard involvedwith raw lithium, cells containing lithium are quite often designed suchthat the lithium becomes exhausted (making the cell inoperative) justbefore other limiting action would occur. Ideally, for precision work, acalibration curve will be made for each design of battery manufactured.By reading the sensing voltage of a cell when the battery is under apredetermined load, and referring to the curve for that type batterywith that load, the remaining life may be read from the curve. (Thisinformation may be printed on the side of the battery for convenience.)For many applications, where only an approximation of the remaining lifeof the battery is desired, the percent the indicated voltage sensed isof the sensed voltage for a new cell is a sufficiently accurate figure.(With the battery under a predetermined load when making the voltagemeasurements, of course.) In FIG. 3 with approximately half the life ofthe cell used, to the voltmeter 71 its as though a short 72 over thisused portion had occurred. In typical embodiments of the invention asdescribed comprising conventional lithium inorganic cell elements suchas having a lithium anode, LiAlCl₄ electrolyte with SOCl₂ depolarizer,and a porous carbon cathode approximately one-half inch in thickness,the voltage drop across the cathode is approximately 50 μV at 0% depthof discharge (new battery) for current densities in the cell of 2milliamps per square centimeter delivered to load 73. Since the cellconfiguration in these embodiments is flat, the remaining cell life issubstantially directly proportional to the measured voltage between thesensing grid and the current collecting grid between the range of 50 μVfor full charge to approaching zero volts for a discharged battery forthe stated predetermined discharge rate. For other cell configurations,such as round, various well known factors of proportionally would existbetween the measured voltage and the remaining cell life.

Generally, the voltage pickup (sensing) grid is fabricated as anintegral part of the porous carbon cathode in a manner similar to thatconventionally used for a conventional carbon cathode current collectiongrid. The voltage pickup grid can be made substantially smaller in alldimensions than the current collecting grid. Substantially no currentflows from the sensing pickup grid, except the minute amount to actuatethe measuring voltmeter. Ideally, the voltmeter should drawsubstantially no current. Generally, conventional, sensitive, high inputimpedance (such as FET) voltmeters are suitable for measurementpurposes.

FIGS. 4, 5, and 6 show respectively detailed pictorial front, side, andtop views of a typical cathode as schematically diagrammed at 11 inFIG. 1. It is a conventional double cathode, i.e., it serves two cells,a right hand cell and a left hand cell. Conventional laminationtechniques are used, with the thickness of the layers depending on gridplacement, to fabricate the novel cathode containing the sensing chargedetermination grid. For example, a conventional mold for a conventionalporous cathode may be used and a carbon slurry placed in the mold insuccessive layers. Ideally, the state of charge sensing grid should beas close to the cathode surface as feasible, therefore the first slurrylayer should be relatively thin, not over approximately 5% of the totalcathode thickness (for two cells). Then the sensing grid is placed inthe mold and the second layer of carbon slurry is poured to the midplane of the total cathode for placement of the current collection grid.The collection grid is positioned and the third layer of slurry thencompletes the cathode. The cathode is then cured in the conventionalmanner. Generally, any fabrication technique used for conventionallyplacing the cathode current collecting grid in the cathode may also beused to place the state of charge sensing grid in the cathode.

It has been found that for conventional size cells and batteries that a25 mesh nickel screen sensing grid made from 0.005 inch wire and rolledto a thickness of approximately 0.005 inches, has sufficient rigidityfor handling ease and an adequately open structure so as to have nodiscernible effect on the conventional performance of the battery cell.Thicker electrodes can, in general, use a screen with a larger wire sizeand a more open mesh. The size and structure of the sensing grid is notcritical. It is generally made much smaller than the current collectorgrid. The actual geometry used in fabricating the sensing grid dependsprimarily on the uniformity of the current distribution in the cellcathode. Generally, in the ideal situation with perfectly uniformcurrent density the size of the sensing grid is based primarily onhandling and fabrication considerations. Theoretically, the sensing gridin the ideal situation could be a single fine wire since appreciably nocurrent is carried by it.

If the conventional current collecting grid, of a particular batteryconstruction to which it is desired to add this improvement, has a largevalue of resistance such that the internal voltage drop within the gridis comparable to that of the drop within the cathode then it isdesirable that the sensing voltage measurement be made from a point onthe current collector where substantially no voltage drop exists, forexample, a point opposite the location where the connection tab isattached to the grid. Referring to FIG. 7, if the grid 80 in cathode 84has a relatively high internal resistance to current flowing toconnection tab 81 then it is desirable to position the sensing grid 82opposite the tab location rather than at a position such as 83 in whichthe resistance of grid 80 would affect the voltage measurement.

In some embodiments of the invention with a particular cell structure itmay be desirable not to utilize the conventional cell current collectinggrid in sensing the state of charge of the cell, but to place two stateof charge sensing grids in the cell as illustrated in FIG. 8. The twosensing grids are of similar construction, as previously described, onegrid 90 is placed near an anode edge 91 of the cathode 92, as inprevious embodiments, and the other sensing grid 93 is positioned nearthe conventional current collecting grid 94 in a position opposite thefirst sensing grid. The voltage measured between th two sensing grids,with a given current flow in the cell, is then a function of the currentdistribution within the cathode which is indicative both of the extentof discharge of the cell, and the extent of remaining life in the cell.

In some relatively recent, but well known, designs of porous electrodeprimary cells, a separate element as a physical separator between thecathode and anode is not necessary nor used. In these structures thecathode and anode materials are adjoining in physical contact. Thecharge sensing grid disclosed herein operates equally well in this typestructure when it is positioned just below the surface of the cathodethat adjoins the anode, as in the previously described embodiments.

I claim:
 1. The improvement in a porous electrode primary batterycomprising a cell having a porous carbon cathode with a surface and anelectrical connector, an anode with a surface, a separator separatingthe said cathode and anode positioned adjacent and between the saidcathode surface and the said anode surface, the said improvement toprovide, while underload, a means for determining the state of charge ofthe said battery, comprising: a voltage sensing grid positioned in thesaid porous cathode near the said surface of the cathode adjacent thesaid separator and means for measuring the voltage between the saidvoltage sensing grid and the said cathode electrical connector.
 2. Theimprovement in a porous electrode primary battery comprising a cellhaving a porous carbon cathode with an electrical connection and anadjoining anode, the said improvement for determining the state ofcharge of the said battery while connected to a load, comprising: avoltage sensing grid, having a connection tab, positioned in the saidporous cathode near the surface adjacent the said adjoining anode forsensing the voltage potential at the location of said sensing gridrelative to the cathode connection potential, whereby the magnitude ofsaid potential is proportional to the said state of charge.
 3. Thestructure as claimed in claim 2 wherein the said cathode has a surfaceadjoining the said anode and the said voltage sensing grid is positionedjust below the said surface of the cathode.